1. Field of the Invention
The present invention relates to a semiconductor memory device and a fabrication method for the same, and more particularly, to an improved semiconductor device fabrication method and a resulting semiconductor memory device having an isolation structure formed by a local oxidation of silicon (LOCOS) method for obtaining a decreased bird's beak and stress.
2. Description of the Conventional Art
In a conventional semiconductor memory device having a LOCOS isolation structure, a channel is formed between a source region and a drain region in the memory device when a predetermined voltage, which is higher than a threshold voltage, is applied to a gate. The magnitude of the threshold voltage required to form the channel is proportional to the thickness of a gate oxide film. Thus, when a field oxide film is about ten times thicker than the gate oxide film, a channel of a transistor provided on a field region is opened by applying a voltage to the gate that is ten times larger than the voltage ordinarily required for turning on the transistor. Thus, an electrical isolation between cells of a semiconductor memory device is sufficient to prevent the transistor from being turned on in response to a voltage ordinarily large enough to turn on the transistor when applied to a gate of an adjacent transistor separated by a field oxide film.
The fabrication method of the conventional LOCOS isolation structure will now be described with reference to FIGS. 1A through 1C.
As shown in FIG. 1A, an upper surface of an Si substrate 11 is oxidized to form a pad oxide film 12 having a thickness of 35 nm. A nitride film 13 having a thickness of 100 nm is deposited on the pad oxide film 12. Nitride film 13 functions as an anti-oxidation film. As further shown in FIG. 1B, a photoresist film 14 is formed and patterned on the nitride film 13. Using the patterned photoresist film 14, the nitride film 13 and the pad oxide film 12 are patterned. Referring to FIG. 1C, the photoresist film 14 is removed and the entire structure is oxidized using a wet oxidation method. During the wet oxidation method, a portion of the substrate 11 covered with the nitride film 13 is not oxidized. By contrast, the exposed surface of the substrate 11 that is not covered with the nitride film 13 is oxidized. As a result of the oxidation of the exposed surface of substrate 11, the field oxide film 15 swells to about an 800 nm thickness, thereby accomplishing an isolation between the cells in accordance with the field oxide film 15.
In the above-described conventional LOCOS isolation structure for a semiconductor memory device, the pad oxide film 12 does not efficiently block the growth of the field oxide film 15 during the oxidation step for forming the field oxide film 15, as shown in FIG. 1C. Consequently, a bird's beak phenomenon is generated in which each end portion of the field oxide film 15 extends under the pad oxide film 12.
In recent years, Y. Sambonsugi et al. published an article entitled, "Oxynitride Pad LOCOS(ON-LOCOS) Isolation Technology for Gigabit DRAMs" in SSDM p. 139, 1995, disclosing a method for decreasing a birds beak by substituting an oxynitride film for the pad oxide film 12. A brief description of that article will now follow.
First, when an Si substrate is nitrated under an atmospheric gas of NH.sub.3 /Ar for 10 minutes at temperatures ranging from 500.degree. C. to 900.degree. C. and then is dry-oxidized for 30 minutes at a temperature of 900.degree. C., a 3 nm thickness of oxynitride film is grown on the substrate. The density of nitrogen in the oxynitride film can be varied by adjusting the temperature during the nitridation. After growing the oxynitride film, a nitride film is deposited. The nitride film is patterned and dry-etched and then the oxynitride film is etched using HF gas. The next steps are identical to those of the conventional fabrication method for a LOCOS isolation structure, as shown in FIG. 1A-1C. As described above, the oxynitride film is employed instead of the pad oxide film. Thus, when the oxidation process is carried out to form the field oxide film, an oxidant diffusion is prevented by the nitride component, which is included in the oxynitride film.
Stated differently, the oxynitride film efficiently blocks the growth of the field oxide film, thereby decreasing the bird's beak.
Moreover, the oxynitride film decreases the bird's beak when compared with the use of an oxide film. However, because the oxynitride film has a thermal coefficient larger than that of the oxide film, stress is increased during the oxidation process used to form the field oxide film.